This invention relates to arrangements for biasing power active devices such as transitors and for changing the bias from moment to moment in response to the signal power being handled by the amplifier or apparatus in which the active device is incorporated.
Active electronic devices such as vacuum tubes, bipolar and field-effect transistors and the like have nonlinear transfer functions. Very often, as with small-signal applications, the signal excursion is sufficiently small so that a portion of the transfer characteristic of the active device can be selected which is approximately linear. The active device is biased to that portion of the characteristic and the signal excursions about that point are sufficiently linear for the intended application. In other applications, it may be necessary to select among active devices to find those which are sufficiently linear for the intended purpose. In still other applications, the nonlinearity of the transfer function can be suppressed by trading gain for linearity, as with degenerative feedback and more recently with feedforward distortion compensation.
Power amplifiers pose a more severe distortion problem than do small-signal amplifiers. Generally speaking, the output signal excursions are large and may encompass a substantial portion of the entire useful characteristic. The internal power dissipation of the power active device is generally relatively large, which forces the physical dimensions of the active device itself (for example, the dimensions of the transistor chip) to be large to allow sufficient area for conducting away the heat to prevent damage to the device. Such larger active devices tend to require compromises in their design which reduces their overall gain. Consequently, the input signal required to drive the active device to produce the rated power output will be larger than in the case of a small-signal device, and thus the signal excursions at the input of the device are larger than in small-signal devices which exacerbates the linearity problem. The relatively low gain of the power devices and their large cost makes feedback and feedforward distortion correction schemes unattractive. Feedback and feedforward may also be disadvantageous for other reasons, as for example, because of high-frequency phase shift considerations. Consequently, the active devices in many applications, as for radio and television transmitters and the like are provided with distortion compensators which predistort the signal applied to the power device to compensate for the expected distortion when the power device is operated at a bias current and voltage level which provides optimum distortion.
It has been discovered, however, that the bias current and voltage of the active device may change during operation in response to the signal level being handled by the device. Thus, the power device deviates from the optimum bias point for which the predistortion was designed, with the result that substantial degradation of the distortion results. It is desirable to compensate for the distortion-causing effect of signal-dependent changes in the operating point of a power device.